The present invention relates to a thermal type flow meter for detecting flow rates of intake and exhaust gas of an engine and its control method.
In a thermal type flow meter as shown in FIG. 1, at least two resistors are disposed in fluid, one of them being used as a temperature measuring resister (CW) for detecting temperatures of the fluid with the other as a heating resister (HW) for detecting flow rates, and the temperature difference (ΔTh) between these resistors is always kept to be constant so as to realize measurement of mass flow rates of the fluid.
This type of thermal flow meter faces a problem that because of the independently separate arrangement of heating resistor and temperature measuring resistor in the fluid, the installation space increases, the pressure loss increases and the production cost is high.
In general, the temperature measuring resistor adapted to detect fluid temperatures is not heated normally by conduction of electric current whereas the heating resistor is set to such a temperature sufficiently higher than a fluid temperature as providing ΔTh=200° C., for example. When a flow rate sensor having the structure as above is used in an exhaust gas atmosphere, an oil component or the like contained in exhaust gas deposits on the sensor surface. If the resistor is at a sufficiently high temperature, the oil component deposited on the sensor surface evaporates and so the sensor can be prevented from being contaminated. But the temperature measuring resistor being at a relatively low temperature cannot evaporate the oil component deposited on its surface and as a result, contaminants cumulate on its surface. With the contaminants cumulated, the fluid temperature cannot possibly be detected accurately and errors in measurement of flow rate will take place.
JP-A-59-136620 describes an expedience in which any temperature measuring resistor not heated by current conduction is unused but two heating resistors are disposed in fluid in order that a flow rate unrelated to fluid temperature is detected from the relation between quantities of heat supplied to the fluid from the respective heating resistors. In the technique disclosed in JP-A-59-136620, the fluid flow rate ρU can be determined from equation (1):
                              ρ          ⁢                                          ⁢          U                =                  C          ⁢                                                    (                                                      I                    1                    2                                    -                                      I                    2                    2                                                  )                            2                                                      (                                                      T                    1                                    -                                      T                    2                                                  )                            2                                                          (        1        )            
wherein
ρ: fluid density, U: fluid velocity,
I1: electric current of heating resister I
I2: electric current of heating resister II
T1: temperature of heating resister I
T2: temperature of heating resister II, and
C: constant value.
As is clear from equation (1), the liquid flow rate is inverse proportional to the square of a difference in temperature between the two heating resistors and therefore, if the temperature difference between the two heating resistors is small, errors are liable to develop in the flow rate obtained from equation (1). It has been considered that for the sake of preventing heat dissipation from the heating resistor to its support member, a plurality of heating resistors may mutually be arranged intimately closely but in such a contrivance, the plural heating resistors are set to be at the same temperature or at very close temperatures. Therefore, in order to utilize the technique described in the above Patent Document, there needs another heating resistor for which temperature setting is discriminatively different from that for the plural heating resistors, raising a problem that the sensor structure becomes complicated.
Further, the technique described in the aforementioned JP-A-59-136620 fails to consider heat dissipation or transfer from the heating resistor to its support member, resulting in a problem that if the heating resistor is at high temperatures and heat dissipation to its support member cannot be negligible vis-à-vis heat dissipation to the liquid, the error in flow rate detection grows.
Incidentally, Japanese Patent No. 2805175 discloses a technique in which a single heating resistor is used to detect a flow rate having no relations with fluid temperatures. In the latter technique, there is provided a switch for bypassing a fixed resistor connected to a bridge circuit and the flow rate unrelated to the temperature of fluid is detected from a change in amounts of heat dissipation from the heating resistor when the switch is turned on and off. The latter technique, however, faces a problem that if the fluid temperature and the flow rate change at higher periods than the period of turn on and off of the switch, accuracy in flow rate detection will be degraded. As known in the art, high-frequency pulsations take place in the course of intake and exhaust of an engine and the error in measurement will possibly grow in the pulsation field as above.